Toshimitsu Morooka

Micro-imaging system using scanning DC-SQUID microscope

A micro-imaging system in a low temperature environment has been developed for the study of superconducting films and magnetic films and for the inspection of superconducting integrated circuits. The system consists of a micro DC-SQUID, a cryostat, a precise scanning stage, and a computer. Two different types of micro DC-SQUIDs were designed. One was a magnetometer (Bz) with a one-turn pick-up coil with a diameter of 10 /spl mu/m, and the other was a gradiometer (dBz/dx) with a planar first-order derivative pick-up coil. Each micro DC-SQUID was integrated on a 3 mm /spl times/3 mm Si chip using thin Nb film fabrication technology. Preliminary experiments were made using the system and several magnetic images were obtained. We present observations of a thin superconducting Nb film pattern by applying the Meissner screening and the magnetic domains of a thin garnet ((YBi)/sub 3/(FeAl)/sub 5/O/sub 12/) film.

Strong, easy-to-manufacture, transition edge x-ray sensor

We developed a membrane structure with a silicon-on-insulator (SOI) wafer by using a micromachining technique to create a transition edge x-ray sensor. In this membrane structure, the part of the SOI layer between the silicon nitride (SiNx) film and the buried oxide layer was etched from the front side to form the SiNx membrane. Advantages of this membrane are that (a) it is stronger than conventional membranes and is therefore suitable for large format arrays, (b) the Si etching time is reduced from 12 h (for conventional etching) to 4 h, and (c) all the fabrication processes are done from the front of the wafer, thus simplifying the manufacturing process.

Observation of Superconducting Device Using Magnetic Imaging System with a Micro-DC Superconducting Quantum Interference Device Magnetometer

A magnetic imaging system with a micro-DC superconducting quantum interference device magnetometer has been developed to evaluate superconducting films and to inspect a superconducting device with a high spatial resolution. A superconducting device, i.e., a DC-SQUID made of a superconducting Nb thin film, was observed using the system. The shape of the device was identified by applying the Meissner effect. The trapped fluxes in the device were clearly visible. In addition, the input current and the shielding current flowing on the device were detected by feeding a current to the device. It was confirmed experimentally that the magnetic imaging system was effective in studying superconducting devices.

Development of Integrated Direct Current Superconducting Quantum Interference Device Gradiometer for Nondestructive Evaluation

A Direct Current Superconducting Quantum Interference Device (DC-SQUID) gradiometer for nondestructive evaluation was developed. A pick-up coil and a DC-SQUID were integrated on a 3×3 mm2 Si chip. In order to raise the spatial resolution of the gradiometer, a coplanar concentric second-order derivative coil was adopted as a pick-up coil. The pick-up coil consisted of a one-turn outer coil and a four-turn inner coil connected in series and counterclockwise to each other. The diameters of the outer coil and the inner coil were 2 mm and 1 mm, respectively. The size of the pick-up coil was determined by calculating the spatial resolution in relation to the bottom thickness of the cryostat. The high spatial resolution of the DC-SQUID gradiometer was experimentally confirmed.

Development of scanning SQUID microscope for studying superconducting films and devices

A micro-imaging system using a low-temperature superconductor (LTS) scanning SQUID microscope (SSM) has been developed for the study of superconducting films and devices. The system consists of a micro DC-SQUID, a cryostat, a precise scanning stage, and a control unit. The spatial resolution was improved by reducing the size of the pick-up coil of the micro DC-SQUID. A cryostat without liquid nitrogen makes the operation of the system easy and speedy. Preliminary experiments were made using the system and several magnetic images were obtained. We present observations of trapped fluxes in two types of superconducting thin film: a single-crystal-like Nb film grown epitaxially and a polycrystalline Nb film. We also present observations of a SQUID array, which consists of 50 DC-SQUIDs connected in series.

Scanning DC-SQUID system with high spatial resolution for NDE

We have developed a scanning DC-SQUID system with a high spatial resolution for NDE. The SQUID system is composed of a concentric multiloop DC-SQUID gradiometer using Nb/AlOx/Nb Josephson junction, a non-magnetic scanning stage, a cryostat and a computer for gathering data and controlling the system. We have developed the concentric multi-loop gradiometer (d/sup 2/Bz/dr/sup 2/, r/sup 2/=x/sup 2/+y/sup 2/) to have a higher spatial resolution and reduce background noise. The gradiometer was integrated on a single 3/spl times/3 mm/sup 2/ Si chip. To reduce the standoff distance between the specimen and the pick-up coil, a cryostat with a thin bottom space was developed. A scanning stage was made of nonmagnetic materials and was driven by air pressure using a rod-less cylinder to avoid making magnetic noise.

Present performance of a single pixel Ti/Au bilayer TES calorimeter

We are developing a superconducting transition-edge sensor (TES) calorimeter for future Japanese X-ray astronomy missions (e.g. NeXT mission). The performance of our single pixel TES calorimeter is presented. We fabricated a Ti/Au (40 nm/110 nm) bilayer TES on a thin silicon-nitride membrane, which is adjusted to have a transition temperature of about 100 mK. The size of the TES is 500μm × 500μm, and 300μm × 300μm gold with a thickness of 300 nm is deposited with sputtering as an X-ray absorber. The TES calorimeter was installed in a dilution refrigerator operated at about 40 mK, with a combination of 400-series SQUID array as an ammeter. Collimated 5.9 keV X-rays (200 um in diameter) from 55Fe isotope were irradiated and X-ray pulses were obtained. Simultaneously with a fast falling time constant of 74.2 us, the energy resolution of 6.6+-0.4 eV was attained, while the baseline noise was 6.4 eV. The contents of the energy resolution are 5.1 eV of the excess noise, 3.3 eV of the readout noise, 1.6 eV of the pulse by pulse variation, and 1.9 eV of the intrinsic noise. The baseline noise are dominated by an unknown excess noise, which increases roughly in proportion to the inverse of the TES resistance. The pulse height is sensitive to the operating conditions, and the superconducting shield appears to have improved it by a factor of about 2. The calorimeter works fine over six months surviving five thermal cycles, even though it is kept in air.

High-current resolution broad-band SQUID amplifier suitable for TES calorimeter

A superconducting quantum interference device (SQUID) amplifier has been developed as a current detector with both high-current resolution and broad bandwidth for a transition edge sensor calorimeter. The amplifier is a two stage SQUID (TSS) that consists of an input-SQUID with a 38-turn input coil and a 100-serial SQUID array (100-SSA) output, and has been integrated on a 3 /spl times/ 3 mm Si chip using Nb thin film fabrication technology. It is designed to increase the amplifier gain and maintain matching with the parameters of the calorimeter. To avoid flux trapping in the SSA, the washer coil of the dc-SQUIDs in the SSA was made with a narrow line width of 17.5 /spl mu/m. We experimentally confirmed that the designed output voltage was achieved using a one-layer p-metal magnetic shield tube in the earths magnetic field. The performance of the shielded TSS amplifier was evaluated in liquid helium. The TSS amplifier had a gain of 10 kV/A and an impedance of 0.07 /spl Omega/ at 100 kHz. When a flux locked loop circuit was used to drive the amplifier, a current resolution of 1 pA//spl radic/Hz and a rise time of 1 /spl mu/s were achieved.

Development of Multilayer Readout Wiring for Large-Format TES X-Ray Microcalorimeter Arrays

Pursuing the feasibility of a large array of transition-edge sensors for future astrophysical missions, we have undertaken a study of multilayer readout wiring. It is composed of thin superconducting signal and return wires made of Al or Nb that sandwich an insulation SiO2 film. Self and mutual inductances between signal and return wires of pixels, and also self fielding of bias leads can be reduced. Also a necessary spatial resource can be half compared to a normal wiring. We have fabricated four types of 20 × 20 wiring samples whose upper and lower wiring widths were 10 and 15 μm. High process yields of 95 ~ 97% were confirmed for all the four arrays in resistance measurements at room temperature. At low temperature, Al-Al samples showed sharp superconducting transitions, low residual resistances 1 ~ 2 mΩ, and high critical currents of >; 100 μA, which suggests that they can be used as our TES readout wiring. On the other hand, Nb-Al samples showed slow transitions and low critical currents ( <; 10 μA). Energy dispersive X-ray spectroscopy measurements indicated that impurities between Al and Nb films at electrical contacts influence these features.

Direct detection of the magnetic flux noise from moving vortices in wide YBa2Cu3O7−δ grain boundary junctions

We measured the flux of a wide bicrystal grain boundary of YBa2Cu3O7−δ film, cooled to 77 K in a field of 10 μT, using a superconducting thin-film coil and Nb-based superconducting quantum interference device (SQUID). When the applied field was changed to, and above, a small threshold value, the sample showed random switching noises with a sharp increase in low-frequency noise power. The results suggested that there was long-distance movement of vortices over 20–500 μm within the grain boundary, driven by the shielding current. The vortex movement was suppressed to lengths of less than 1 μm in a slotted grain boundary by making an array of SQUIDs that can hold vortices stably.